The one-way gates allowed water to flow out, but little or no water to flow in, reducing the natural flushing of the river with salt water from the harbor. The result was a habitat increasingly hostile to marine life.

After 1970, “it got so dirty that it was like coffee,’’ said Bianchi, who spent childhood summers at the river. Her parents owned a two-acre island - since passed down to her brothers - in the heart of the affected area.

With the water no longer inviting to swimmers, the family stopped making regular trips. The tide gates, it seemed, had put a bleak coda on more than 300 years of human tinkering with the Green Harbor River.

In the last two years, though, and since last summer in particular, the river has been slowly coming back to life.

Thanks to the replacement in 2009 of one of the four “flapper’’ gates - which closed as the tide tried to come in - with an adjustable gate, more salt water from the harbor is flowing into the river, flushing it out and boosting oxygen and salinity. Fresh-water plants that have choked the river are beginning to recede, and some of the natural marine life is returning.

“I think we’re all surprised at how quickly we’re seeing bigger benefits than we anticipated,’’ he said.

Normally, oxygen in the water on the inland side of the gates at Dyke Road becomes so depleted over the summer, that by August fish start to die. The water gets murky, and algae float on the surface in big mats of green and brown. When the algae die, they settle on the bottom and decompose, further degrading water quality.

Old postcard of the Green River Harbor and brothers Eric and Dave Blauss enjoying the river in 1965.

But last summer, Wennemer saw fewer algae, no fish dying, and clearer water. Most surprisingly, he saw a marked reduction of phragmites, the tall, tassel-topped reeds that thrive in fresh and brackish wetlands. In recent years, phragmites had begun filling the river from the sides.

Bianchi began studying the river in 2003, long before the restoration project began. A science teacher at Whitman-Hanson Regional High School, she has brought students to the river to measure indicators of water quality. At first, she had her doubts that anything beyond education would come of it.

“I said a prayer in the beginning, because my brother had been trying for years to get people in Marshfield interested,’’ she said.

This time, the stars aligned. Her work built momentum for the project at the state level. Today, Jason Burtner, South Shore regional coordinator for the Office of Coastal Zone Management, serves as project manager.

The tide gate and water monitoring received funding from a variety of state, federal, and private sources. State sources include the Office of Coastal Zone Management, the Massachusetts Bays Program, and the Division of Ecological Restoration. The Corporate Wetlands Restoration Partnership contributed, and the Marshfield Yacht Club was required to donate about $20,000 as mitigation for dredging the South River.

Employees of the Marshfield Department of Public Works raise and lower the gate, in response to decisions made by an informal committee of interested parties. They opened it a small amount the first year, and then more last summer. The gate can be closed during a storm, as it was for Tropical Storm Irene.

Not only are the reeds dying and the water clearer, Burtner said, but the temperature has dropped. The salinity and dissolved oxygen are higher, and fish are able to swim upstream into the river. Worms and clams are returning, too.

Burtner, Wennemer, and Sara Grady of the Massachusetts Bays Program paddled the river recently to check out its progress. They saw phragmites stunted from salt, and when the sun was right, they could see the bottom.

“This is just a really great success story for what was a relatively small financial investment,’’ Burtner said.

The cost was $149,400, not including the town’s contribution, he said. Wennemer pegged Marshfield’s contribution at just over $40,000.

Grady said students from the Cohasset Center for Student Coastal Research were involved in testing the water over the summer, and the tests showed a greater variety of invertebrates present upstream from the dike, compared with the previous year.

The river and harbor are getting closer in terms of the composition of vegetation, fish, and other animals, she said.

According to Wennemer, fishermen have reported catching flounder and striped bass upstream from the dike, something virtually unheard of in the years leading up to the project.

Despite its early success, work to restore the river is far from finished. Officials plan to continue monitoring the height of the water on the inland side of the dike and evaluating how much inflow they can allow without flooding nearby homes.

Neighbors who live near the river expressed some concern before the adjustable gate was installed, Wennemer said, but he has not received any complaints since the gate began operating.

Eventually, the useful life of the remaining flapper gates will expire, he said, and the town and other funders will have to decide what to install in their place, and how much to restrict the tide’s natural movement.

While he doesn’t know what they’ll decide, Wennemer does know what has already been achieved by the new gate.

“The increase in water flow up there has tremendously improved the water quality,’’ he said.

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